OTTAWA—The parliamentary budget office says a pledge by the Trudeau Liberals to plant two billion trees could cost almost double what the government says. The tree-planting spree, spread over a decade, is supposed to start in the spring and cost $3.16 billion over that time, based on federal estimates. Getting to the 2030 target means planting about 200 million trees a year more than the usual 600 million or so. The spending watchdog’s analysis suggests getting there is also going to require more money, about $2.78 billion more, bringing the overall cost closer to $5.94 billion. The budget officer’s report is based on a similar program the Ontario government ran, using the average per-tree cost and adjusting for inflation over the 10-year planting period. The tree-planting promise made during the 2019 federal election was part of a $3-billion, decade-long effort aimed at managing, conserving and restoring the country’s forests, grasslands …